The results follow a day of violent clashes between Spanish national police and supporters of the disputed vote. Hundreds were injured as riot police raided polling stations and fired rubber bullets in a concerted attempt to deny the vote legitimacy.

The vote risks plunging the country into one of its worst political crises since the end of Gen. Francisco Franco's dictatorship in 1975.

Catalonia has its own regional government, with considerable powers over healthcare, education and tax collection. But Catalonia has long complained that its revenues subsidize other parts of Spain. Catalan nationalists argue the region is a separate nation with its own history, culture and language, and that it should have increased fiscal independence.

Health services raises number of people injured by police Sunday to 761, with two seriously injured.

Barcelona's deputy mayor said police fired rubber bullets at people as they attempted to vote in the referendum, which Spain's top court has declared illegal.

"Police are confiscating ballot boxes to respect the judicial mandate and the law regarding the illegal referendum," Spain's interior ministry said on Twitter Sunday.

In a tweet, the Catalan administration called on the Spanish government representative in the province to resign.

Catalonia's separatist government has remained adamant that the vote on independence would go ahead. Many schools designated as polling stations were occupied overnight in an attempt to keep them open on referendum day.

"This moment means a lot to me," Joana Rauet, 89, told CNN after casting her vote at Josep Maria Jojol school in Barcelona Sunday. "I feel satisfied that I was able to take part. I'm feeling very happy," she said. Voters were clapped and hugged by a waiting crowd as they left the school, having cast their ballot. People told CNN they had been told to stay in case the police arrived to shut the voting station down.

"If the police show up, I will stand my ground. I will peacefully resist," Xan Fernando, 20, a student told CNN.

The dispute between the regional government in Barcelona and the Spanish government has become increasingly bitter in recent weeks, with mass protests held across the region.

In the runup to the vote, national authorities seized ballot papers, voter lists and campaign material, as well as sending thousands of extra national police to the region.

High-ranking Catalan officials involved in organizing the referendum were arrested.

In the past few days, authorities blocked the use of a voting location app and seized vote-counting software.

Puigdemont, who called the referendum in June, had urged voters to go to the polls Sunday despite Madrid's opposition.

The 5.3 million voters on the electoral roll were being asked to respond yes or no to the question: "Do you want Catalonia to be an independent state, in the form of a Republic?"

Polling stations are due to close at 8 p.m. local time on Sunday. Results are expected around 10 p.m. local time (4 p.m. ET).

Why is the referendum taking place?

Catalonia, a wealthy region in Spain's northeast, has its own regional government -- or Generalitat -- which already has considerable powers over healthcare, education and tax collection.

But Catalan nationalists want more, arguing that they are a separate nation with their own history, culture and language and that they should have increased fiscal independence.

The region pays tax to Madrid, and pro-independence politicians argue that complex mechanisms for redistributing tax revenue are unfair on wealthier areas and result in Catalonian revenues subsidizing other parts of Spain.

Others, including Spanish Prime Minister Mariano Rajoy, insist that the country cannot be divided.

Catalonia's campaign to break away has been gaining momentum since 2010, when Spain's economy plunged during the financial crisis. Catalonia held a symbolic poll in 2014, in which 80% of voters backed complete secession -- but only 32% of the electorate turned out.

The Catalan government has not yet made clear how it will respond in the event of a "yes" vote.